Oxide coated cutting insert

ABSTRACT

A cutting tool insert includes a body of a hard alloy of cemented carbide, cermet, ceramics, cubic boron nitride based material or high speed steel. A hard and wear resistant coating, having at least one layer, to which an (Al,Cr) 2 O 3  layer is applied. This insert is particularly useful for machining of steel and stainless steel. The coating with a total thickness of 2-20 μm has one or several layers, at least one of which is an (Al,Cr) 2 O 3  layer with a thickness of 1-5 μm having a corundum phase crystalline structure and a composition (Al 1-y Cr y ) 2 O 3  with 0.4≦y≦0.6. The (Al,Cr) 2 O 3  layer has a fiber texture with rotational symmetry in the direction of the coated surface normal to an inclination angle, φ, of the basal planes relative to the coated surface normal or the inclination angle, φ, for the highest peak in the pole plot with 70° &lt;φ&lt;90° .

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a tool for machining by chip removalcomprising a body of a hard alloy of cemented carbide, cermet, ceramics,cubic boron nitride based material or high speed steel and a hard andwear resistant oxide designed to be used in machining of steel andstainless steel, preferably at high cutting speeds. The said coating iscomposed of one or more layers of which at least one layer is a texturedphysical vapour deposited (PVD) corundum phase alumina containingchromium (Al,Cr)₂O₃.

2. Decscription of the Related Arts

Textured α-Al₂O₃ layers, produced with chemical vapour deposition (CVD)are disclosed in, e.g., EP 603144, EP 1528125, EP 1477581, EP 1655387,EP 659903, EP 738336, EP 1655388, EP 1655392, US 2007/104945, US2004/202877.

EP 1479791 discloses a cutting tool composed of cemented carbide orcermet, and a hard coating; wherein the hard coating includes an α-Al₂O₃layer formed by CVD, with the highest peak, measuring the inclination ofthe α-Al₂O₃ basal planes relative to the normal of the surface within arange of 0-10 degrees as determined by electron back scatteringdiffraction (EBSD).

EP 744473 discloses textured γ-Al₂O₃ layers produced by PVD.

U.S. Pat. No. 5,310,607 discloses a hard coating including (Al,Cr)₂O₃crystals and a chromium content higher than 5 at % wherein the(Al,Cr)₂O₃ is a single crystal. The coating is deposited at atemperature lower than 900 C. The hard coating is deposited by a CVD orPVD process.

When machining steels with an alumina coated cemented carbide tool, thecutting edge is worn according to different wear mechanisms, such aschemical wear, abrasive wear, adhesive wear and by edge chipping causedby cracks formed along the cutting edge. The domination of any of thewear mechanisms is determined by the application, and is dependent onproperties of the machined material, applied cutting parameters and theproperties of the tool material. In general, it is very difficult toimprove all tool properties simultaneously, and commercial cementedcarbide grades have usually been optimised with respect to one or few ofthe above mentioned wear types, and have consequently been optimised forspecific application areas. This can, for instance, be achieved bycontrolling the texture of the alumina layer.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a wear resistant andhard oxide coated cutting tool with enhanced performance for machiningof steel and stainless steel.

The cutting tool insert according to the present invention includes abody of a hard alloy of cemented carbide, cermet, ceramics, cubic boronnitride based material or high speed steel comprising a textured oxidelayer of corundum phase (Al,Cr)₂O₃ with excellent metal machiningproperties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows a schematic view of the hexagonal crystal structure withthe a-axis (100), b-axis (010) and c-axis (001) marked.

FIG. 1 b shows a schematic view of the fibre texture with (S) coatingsurface and (φ) inclination angle of the c-axis (001) of the hexagonalstructure (FIG. 1 a) and the normal (n) to the coating surface.

FIG. 2 shows a schematic side view of the deposition chamber showing (1)vacuum chamber, (2 a) cathode material A, (2 b) cathode material B, (3)fixture, (4) power supply for biasing, (5 a) cathodic arc power supply(5 b) cathodic arc power supply, (6) inlet for process gas and (7)outlet for vacuum pump.

FIG. 3 shows a scanning electron micrograph in secondary mode of afractured cross section of a coating according to the invention. (A)body, (B) bonding layer, (C) (Al,Cr)O layer, (D) (Al,Cr)N layer and (E)TiN layer.

FIG. 4 shows an x-ray diffraction pattern of a textured (Al,Cr)₂O₃layer. The peaks of cemented carbide are marked with solid lines whereasthe peaks originating from (Al,Cr)₂O₃ with dashed lines.

FIG. 5 shows (A) (001) pole figure and (B) (001) pole plot graph of a(Al,Cr)₂O₃ layer according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, there is provided a cutting tool formachining by chip removal, particularly useful in metal cutting of steeland stainless steel, comprising a body of a hard alloy of cementedcarbide, cermet, ceramics, cubic boron nitride based material or highspeed steel onto which a coating is deposited comprising:

preferably a first (innermost) bonding layer (FIG. 3(B)) of, e.g., TiNor (Al,Cr)N preferably less than 0.5 μm according to prior art.

a layer of (Al_(1-y)Cr_(y))₂O₃ with 0.4≦y≦0.6, preferably y=0.5, with athickness of 0.5-10 μm, preferably 1-5 μm, most preferably 2-4 μm, withtextured columnar grains. The (Al,Cr)₂O₃ layer has a corundum structureformed by PVD and a fiber texture with rotational symmetry in thedirection of the coated surface normal with an inclination angle, φ,(FIG. 1 b) of the basal planes relative to the coated surface normal(FIG. 5A) or the inclination angle, φ, for the highest peak in the poleplot (FIG. 5 B) with 70°<φ<90°, preferably 80°<φ<90° as determined by,e.g., electron back scattering diffraction (EBSD) or x-ray diffraction(XRD).

Said (Al,Cr)O layer has a compressive stress level of −4.5<σ<−0.5 GPa,preferably of −3.0<σ<−1.0 GPa.

The composition, y, of (Al_(1-y)Cr_(y))₂O₃ is determined by, e.g., EDSor WDS.

Said body may further be coated with an inner single- and/or multilayercoating of, e.g. TiN, TiC, Ti(C,N), (Al,Cr)N or (Ti,Al)N, preferably(Ti,Al)N, (Al,Cr)N, and/or an outer single- and/or multilayer coatingof, e.g. TiN, TiC, Ti(C,N), (Al,Cr)N or (Ti,Al)N, preferably (Ti,Al)N,(Al,Cr)N, to a total thickness 1 to 20 μm, preferably 1 to 10 μm andmost preferably 2 to 7 μm according to prior art.

The deposition method for the layer of the present invention is based oncathodic arc evaporation of an alloy or composite cathode under thefollowing conditions; (Al,Cr)₂O₃ layers are grown using Al+Cr-cathodeswith a composition between (40 at % Al+60 at % Cr) and (60 at % Al+40 at% Cr) and preferably between (45 at % Al+55 at % Cr) and (55 at % Al+45at % Cr). The evaporation current is between 50 A and 200 A depending onthe cathode size and preferably between 60 A and 90 A using cathodes of63 mm in diameter. The layers are grown in an Ar+O₂ atmosphere,preferably in a pure O₂ atmosphere at a total pressure of 1.0 Pa to 5.0Pa, preferably 1.0 Pa to 2.5 Pa. The bias is −100 V to −250 V,preferably −125 V to −175V. The deposition temperature is between 300°C. and 550° C., preferably between 350° C. and 450° C.

The invention also relates to the use of cutting tool inserts accordingto the above for machining of steel and stainless steel at cuttingspeeds of 75-600 m/min, preferably 150-500 m/min, with an average feed,per tooth in the case of milling, of 0.08-0.5 mm, preferably 0.1-0.4 mmdepending on cutting speed and insert geometry.

EXAMPLE 1

Grade A: Cemented carbide inserts with the composition 10 wt % Co andbalance WC, were used.

Before deposition, the inserts were cleaned in ultrasonic baths of analkali solution and alcohol. The system was evacuated to a pressure ofless than 2.0×10⁻³ Pa, after which the inserts were sputter cleaned withAr ions. At first, a bonding layer of TiN with a thickness of 0.2 μmfollowed by a textured (Al,Cr)₂O₃ layer of thickness 2.5 μm, were grownby cathodic arc evaporation of an alloyed (50 at % Al+50 at % Cr)cathodes, 63 mm in diameter (position (2 a) and (2 b) in FIG. 2 a) in99.995% pure O₂ atmosphere at a total pressure of 1.5 Pa and adeposition temperature of about 400° C. to a total coating thickness of3 μm. The evaporation current was 75 A and the bias was held at −150 Vduring depositions. Finally, a top colour coating consisting of 0.3 μm(Al,Cr)N and 0.2 μm TiN was applied.

A fractured cross-section SEM micrograph of the coating is shown in FIG.3 with (A) body, (B) bonding layer, (C) (Al,Cr)O layer, (D) (Al,Cr)Nlayer and (E) TiN layer.

The XRD patterns of the as-deposited layers were obtained usingCuKa-radiation and a θ-2θ configuration. FIG. 4 shows the XRD pattern ofa coating according to the invention with a textured corundum phase(Al,Cr)₂O₃ layer. The peaks originating from the (Al,Cr)₂O₃ layer aremarked with dotted lines whereas the peaks of cemented carbide aremarked with solid lines

The EBSD pole figure (FIG. 5(A)) and pole plot graph (FIG. 5(B)) of theas-deposited corundum phase (Al,Cr)₂O₃ layers in the c-axis (001)direction (FIG. 1 a), respectively, showing a fiber texture (rotationalsymmetry) in the direction of the coated surface normal (FIG. 1 b) withan inclination angle, φ (FIG. 1 b), of the basal planes relative to thecoated surface normal between 70 and 90°. The highest peak in the poleplot is close to 90°. The EBSD data were obtained using a LEO Ultra 55scanning electron microscope operated at 20 kV equipped with a HKLNordlys II EBSD detector and evaluated with the Channel 5 software.

The residual stresses, σ, of the (Al,Cr)₂O₃ layer was evaluated by XRDmeasurements using the sin²ψ method. The measurements were performedusing CrKα-radiation on the (Al,Cr)₂O₃ (116)-reflection. The residualstress value was 2.4±0.3 GPa as evaluated using a Possion's ratio ofv=0.26 and Young's modulus of E=420 GPa.

The composition, y=0.61, of (Al_(1-y)Cr_(y))₂O₃ was estimated by energydispersive spectroscopy (EDS) analysis using a LEO Ultra 55 scanningelectron microscope with a Thermo Noran EDS detector operating at 10 kV.The data were evaluated using a Noran System Six (NSS ver 2) software.

EXAMPLE 2

Grade B: A layer of 3.0 μm Ti_(0.34)Al_(0.66)N was deposited by PVD oncemented carbide inserts with the composition 10 wt % Co and balance WC,according to prior art.

EXAMPLE 3

Grade C: A coating consisting of 3.0 μm Ti(C,N)+3 μm α-Al₂O₃ wasdeposited by CVD on cemented carbide inserts with the composition 10 wt% Co and balance WC, according to prior art.

EXAMPLE 4

Grade D: Example 1 was repeated using cemented carbide inserts with thecomposition 5 wt % Co and balance WC.

EXAMPLE 5

Grade E: A layer of 3.0 μm Ti_(0.34)Al_(0.66)N was deposited by PVD oncemented carbide inserts with the composition 5 wt % Co and balance WC,according to prior art.

EXAMPLE 6

Grade F: A coating consisting of 3.0 μm Ti(C,N)+3.0 μm α-Al₂O₃ wasdeposited by CVD on cemented carbide inserts with the composition 5 wt %Co and balance WC, according to prior art.

EXAMPLE 7

Grades A, B and C were tested in machining in steel.

Operation Face milling Cutter diameter 125 mm Material SS1672 Inserttype SEEX1204AFTN-M15 Cutting speed 300 m/min Feed  0.2 mm/tooth Depthof cut  2.5 mm Width of cut 120 mm Results Tool life (min) Grade A(grade according to invention) 6.1 Grade B 5.9 Grade C 3.4

The test was stopped at the same maximum flank wear. The wear resistancewas much improved with the grade according to the invention.

EXAMPLE 8

Grades A, B and C were tested in machining in stainless steel.

Operation Shoulder milling Cutter diameter   32 mm Material SS1672Insert type XOEX120408-M07 Cutting speed  275 m/min Feed 0.25 mm/toothDepth of cut   3 mm Width of cut  8.8 mm Results Tool life (min) Grade A(grade according to invention) 5.1 Grade B 4.1 Grade C 2.3

The test was stopped at the same maximum flank wear. The wear resistancewas much improved with the grade according to the invention.

EXAMPLE 9

Grades D, E and F were tested in machining in stainless steel.

Operation Interrupted turning Material SS2348 Insert type CNMG120408-MR3Cutting speed  80 m/min Feed 0.3 mm Depth of cut   2 mm Results Toollife (cycles) Grade D (grade according to invention) 5 Grade E 2 Grade F4

The test was stopped at the same maximum flank wear. The wear resistancewas much improved with the grade according to the invention.

EXAMPLE 10

Grades D, E and F were tested in machining in steel.

Operation Interrupted turning Material SS1672 Insert type CNMG120408-MR3Cutting speed 350 m/min Feed  0.3 mm Depth of cut  2 mm Results Toollife (min) Grade D (grade according to invention) 9.5 Grade E 4.1 GradeF 9.2

The test was stopped at the same maximum flank wear. The wear resistancewas much improved with the grade according to the invention.

The invention claimed is:
 1. A cutting tool insert comprising: a body ofa hard alloy of cemented carbide, cermet, ceramics, cubic boron nitridebased material or high speed steel; a hard and wear resistant coatingapplied to the body, comprising one or several layers, at least one ofwhich is an (Al,Cr)₂O₃ layer, wherein said layer has a corundum phasecrystalline structure and a composition (Al_(1-y)Cr_(y))₂O₃ with0.4≦y≦0.6, with a thickness of 0.5 to 10 μm and a fiber texture,rotational symmetry, in a direction of a coated surface normal with aninclination angle, φ, of basal planes relative to the coated surfacenormal is 70° <φ<90° or an inclination angle, φ, for a highest peak in apole plot is 70° <φ, <90°.
 2. The cutting tool insert according to claim1, wherein said inclination angle, φ, of the basal planes relative tothe coated surface normal is 80° <φ<90°.
 3. The cutting tool insertaccording to claim 1, wherein said layer has a residual stress of−−4.5<φ<−0.5 GPa.
 4. The cutting tool insert according to claim 1,wherein said layer has been deposited with pressure vapour deposition.5. The cutting tool insert according to claim 1, wherein said body iscoated with an inner single- and/or multilayer coating of, TiN, TiC,Ti(C,N), (Al,Cr)N or (Ti,Al)N, and/or an outer single- and/or multilayercoating of, TiN, TiC, Ti(C,N), (Al,Cr)N or (Ti,Al)N, to a total coatingthickness of 1 to 20 μm.
 6. The cutting tool insert according to claim1, wherein the (Al,Cr)₂O₃ layer was deposited by cathodic arcevaporation using Al +Cr-cathodes with a composition between (40 at %Al+60 at % Cr) and (60 at % Al+40 at % Cr), an evaporation currentbetween 50 A and 200 A depending on the cathode size in an Ar+O₂atmosphere, at a total pressure of 1.0 Pa to 5.0 Pa, a bias of −100 V to−250 V, and a deposition temperature of between 300° C. and 550° C.
 7. Amethod of cutting with the cutting tool insert according to claim 1,comprising: machining steel or stainless steel at cutting speeds of75-600 m/min, with an average feed, per tooth when milling, of 0.08-0.5mm, depending on cutting speed and insert geometry.
 8. The cutting toolinsert according to claim 1, wherein y=0.5.
 9. The cutting tool insertaccording to claim 1, wherein said layer has a thickness of 1-5 μm. 10.The cutting tool insert according to claim 1, wherein said layer has aresidual stress of −3.0<φ<−1.0 GPa.
 11. The cutting tool insertaccording to claim 1, wherein said layer was deposited by cathodic arcevaporation.
 12. The cutting tool insert according to claim 1, whereinsaid body is coated with an inner single- and/or multilayer coating of,(Ti,Al)N or (Al,Cr)N, and/or an outer single- and/or multilayer coatingof (Ti,Al)N or (Al,Cr)N, to a total coating thickness of 1 to 10 μm. 13.The cutting tool insert according to claim 1, wherein the (Al,Cr)₂O₃layer was deposited by cathodic arc evaporation using Al+Cr-cathodeswith a composition between (45 at % Al+55 at % Cr) and (55 at % Al+45 at% Cr), an evaporation current between 50 A and 200 A depending on thecathode size in a pure O₂ atmosphere, at a total pressure of 1.0 Pa to2.5 Pa, a bias of −125 V to −175 V, and a deposition temperature ofbetween 350° C. and 450° C.
 14. The method according to claim 7, whereinthe average feed per tooth is 0.1-0.4 mm.